In recent years, non-thermal plasma generated in a packed bed reactor has been shown to be effective in reducing nitric oxides (NOx) produced by power plants and standby generators. These units usually have a reducing agent, such as urea, to enhance the conversion efficiency. The packed bed reactor consists essentially of a high voltage center electrode inserted into a cylinder of dielectric material, usually a form of glass or quartz.
An outside or ground electrode is formed by a coating of metal in various forms, including tape, flame spray, mesh, etc. The space between the center electrode and the inside diameter of the dielectric tube is filled or packed with small diameter glass beads. When high voltage alternating current is applied to the center electrode, the surfaces of the beads go into corona, producing a highly reactive and selective surface for inducing the desired reaction in the gas.
Unfortunately, the packed bed design with its loose beads and glass dielectric is impractical for use in the conditions found in a mobile emitter, such as a car or truck. The vibration and wide temperature swings of the vehicle system would damage the packed bed and the necessary temperature and vibration isolation needed to make it survive would not be cost effective.
A stacked plate reactor for use with diesel engines and other engines operating with lean air fuel mixtures is disclosed in commonly assigned U.S. patent application Ser. No. 09/465,073, filed Dec. 16, 1999, entitled “Non-thermal Plasma Exhaust NOx Reactor,” which is hereby incorporated by reference herein in its entirety. Disclosed therein is a reactor element comprising high dielectric, nonporous, high temperature insulating means defining a group of relatively thin stacked cells forming gas passages and separated by the insulating means. Alternate ground and charge carrying electrodes in the insulating means on opposite sides of the cells are disposed close to, but electrically insulated from, the cells by the insulating means. The electrodes may be silver or platinum material coated onto alumina plates. Conductive ink is sandwiched between two thin nonporous alumina plates or other suitable insulating plates to prevent arcing while providing a stable electrode spacing for a uniform electric field. The electrodes are coated onto alumina in a pattern that establishes a separation between the electrodes and the connectors of alternate electrodes suitable to prevent voltage leakage.
In commonly assigned U.S. patent application Ser. No. 09/511,590, filed Feb. 23, 2000, entitled “Design and Method of Manufacturing a Plasma Reactor for Treating Auto Emissions—Stacked Shapes,” which is also hereby incorporated by reference herein in its entirety, a non-thermal plasma reactor element is prepared from a planar arrangement of formed building blocks of dielectric material. The formed shape defines an internal cell in the plasma reactor having an exhaust passage for flowing exhaust gas to be treated therethrough. Individual cells are provided with a conductive print disposed thereon to form electrodes and connectors. In a preferred embodiment, the conductive print comprises a continuous grid pattern having a cutout region disposed opposite the terminal connector for reducing potential charge leakage. Multiple cells are stacked and connected together to form a multi-cell stack.
Commonly assigned U.S. patent application Ser. No. 09/517,681, filed Mar. 2, 2000, entitled “Plasma Reactor Design for Treating Auto Emissions—Durable and Low Cost” which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma reactor element for conversion of exhaust gas constituents. The reactor comprises an element prepared from an extruded monolith of dense dielectric material having a plurality of channels separated by substantially planar dielectric barriers. Conductive material printed onto selected channels forms conductive channels that are connected along bus paths to form an alternating sequence of polarity, separated by exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and for preventing electrical leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged. During operation, exhaust gas flows through exhaust channels and is treated by the high voltage alternating current flowing through the conductive channels. The planar shape of the dielectric barriers provides a uniform electrical response throughout the exhaust channels.
Commonly assigned U.S. patent application Ser. No. 09/517,682, filed Mar. 2, 2000, entitled “Method Of Manufacture Of A Plasma Reactor With Curved Shape For Treating Auto Emissions,” which is hereby incorporated by reference herein in its entirety, discloses a non-thermal plasma rector element wherein a swept shape substrate is formed and treated to create the non-thermal plasma reactor element. The substrate is formed via extrusion providing a series of nested, concentric dielectric barriers. Selected channels are coated with conductive material to form conductor channels for forming an electric field around exhaust channels. Conductive channels and channels not selected for exhaust flow are plugged at end portions of the monolith with a material suitable for excluding exhaust gases and preventing electrical charge leakage between conductive channels. Exhaust channels, disposed between opposite polarity conductive channels, are left uncoated and unplugged.
While the above non-thermal plasma reactors meet some of the current needs and objectives, additional problems remain to be solved to achieve improved reactor and volumetric efficiency, simplified assembly, and reduced cost. For example, current stacked planar reactors have a parting line when stacked that lies in the same plane as the metal electrode print. Due to the finite thickness of the metal electrode print and camber or thickness variation that may be present in one or both of the dielectric layers, there is a resultant gap between the layers. When the reactor is energized with high voltage, there is a tendency for charge to leak through this gap to the nearest ground path, causing thermal arcing. In an effort to solve this problem, current designs provide a separation (typically about 19 millimeters) between the edge of the electrode and the edge of the dielectric layer. Unfortunately, the potential active area of the electrode is effectively reduced by the amount of the separation.
In addition, current stacked planar reactors require substantial fixturing to align pieces during assembly. Planar reactors using metallized plates and discrete spacers require fixturing to hold each spacer in place relative to the metallized plates during assembly. Formed c-shapes and box shapes solve some of these problems by eliminating the need for spacers and are less expensive to assemble. However, some fixturing is required to align the shapes into the stack.
Further, stacked planar reactors rely upon a stack of substrates or shapes that determine the overall height. Since each layer has a thickness variation and camber tolerance, electrode print thickness variation, and possibly burrs, there is a substantial potential variation in stack height. These issues complicate canning the reactor substrate to withstand severe applications, such as automotive after-treatment, since variation that exceeds about 10 millimeters is typically accommodated by custom sizing or other expensive canning methods.
Extruded monolithic substrates used as the building blocks for the reactor element are not prone to the parting line gap, excessive height variation, or excessive fixturing and handling problems. However, structural webs or ligaments within plasma channels, which may be employed to impart structural integrity, can potentially adversely affect constituent conversion efficiency.
There remains a need in the art for an improved non-thermal plasma reactor providing reactor and volumetric efficiency, simplified assembly, and reduced overall cost.